Illustrating new understanding of adsorbed water on silica for inducing tetrahedral cobalt(II) for propane dehydrogenation

Highly dispersed metal sites on the surface of silica, achieved from immobilization of metal precursor within hydroxyl groups, has gained increasing attention in the field of heterogeneous catalyst. However, the special role of adsorbed water derived by hydroxyl groups on the silica is generally ignored. Herein, a new understanding of adsorbed water on the formation of highly dispersed tetrahedral Co(II) (Td-cobalt(II)) sites is illustrated. It is indicated that sufficient adsorbed water induces the transformation of precursor of Co(NO3)2 into intermediate of [Co(H2O)6]2+. Subsequently, [Co(H2O)6]2+ makes the highly dispersed Td-cobalt(II) sites to be available during direct H2-reduction process. A systematic characterization and DFT calculation prove the existence of the adsorbed water and the importance of the intermediate of [Co(H2O)6]2+, respectively. The as-synthesized catalyst is attempted to the propane dehydrogenation, which shows better reactivity when compared with other reported Co based catalysts.


Reviewer #1 (Remarks to the Author):
I read the manuscript with great interests. Authors described the role of water during the synthesis of Co-based catalyst. The results showed the hydrates is critical to the tetrahedral Co site formation. The as-synthesized catalyst was attempted to the PDH and results were encouraging. The topic is of great importance as the synthesis of catalyst is utmost crucial to determine the catalytic activity. Any details such as the role of water regrading the understanding on catalyst synthesis should draw attention to the community. Authors attempted to show the results of water role, nonetheless, the discussion seems poor, for instance, how the fully coordinated Co(H2O)6 can coordinate with hydroxyls without the consideration of electrostatic interaction which I think is more important in terms of stabilizing the precursors. In other words, the discussion is not fully convincing at this moment, the calculation is also not that detailed to understand. All the figures should be of high resolution.

Reviewer #3 (Remarks to the Author):
In this paper, the authors prepared the catalyst with highly dispersed Td-cobalt(II) on the surface of silica, achieving high catalytic performance of propane dehydrogenation by inducing adsorbed water. In addition, a systematic 1. In the line 172, the authors claimed that "in situ XPS measurement from Fig. S5 confirmed again that only Co(II) species were found on the Dir-reduction sample." However, Fig. S5 is not a in situ X-ray photoelectron spectroscopy. How to define this figure need to reconsider.
2. In the line 184, the authors mentioned that "… Co was connected strongly within the silica support and hard to be reduced." However, it is better to use the word of "Co species" to replace the word of "Co" if the authors do not want to mention the valence state of cobalt component, because the latter will lead readers to consider the Co species are metallic Co.
3. According to the result of H2-TPR (Fig. 1c), the authors said "… all of the Co oxides on the surface were reduced by H2." Therefore, the authors may need to explain that the high content of Co 2+ of the H2-reduction catalyst in Fig. 1b. 4. The notes of Fig. 1c, "Co3O4→CoO→Co" and "Co 2+ ", should be unified.
5. In the line 231, the authors said "then mixed physically with Co(NO3)2·6H2O in glove box before conducting H2-direct reduction (Dir-reduction (PM200))". In the line 235 the authors said "Afterwards, 900 o C-calcined SBA-15 was mixed physically with Co(NO3)2·6H2O to obtain Dir-reduction (PM900) sample". These ambiguous sentences would confuse audiences whether the PM200 was reduced in H2-direct reduction process or not.
6. In the lines 257-261, the authors shown the H2-TPR curves of PM200, PM900 and IMP900catalyst, which revealed that Co3O4 is in the catalysts. Thus, XPS spectra of these catalysts should have the peak of Co 3+ , while it is absent in Fig. 2c. 7. The reduction temperature of contrast samples (PM200, PM900 and IMI900) should be presented. The Co/SBA-15 is reduced 600 o C (line 538), if the contrast samples were also reduced at 600 o C, the authors should explain why in the H2-TPR profiles ( Fig. 2b) reduction peaks occur below 600 o C.
8. The authors should explicit the function of the H2 reduction process, because the authors say H2 reduction process is to generate highly dispersed Td-cobalt(II) sites in the abstract, while the authors also say that this process is to induce adsorb water in the line 106. Thus, the authors may need to explain it.
9. In addition, it is better to enlarge the font size of some icons, like Fig. 1(a , b, c, d, e, f).
10. Please specify the manufacturers and purity of all the chemicals and materials used in this work.

Reviewer #1
Comment (1): Any details such as the role of water regrading the understanding on catalyst synthesis should draw attention to the community. Authors attempted to show the results of water role, nonetheless, the discussion seems poor, for instance, how the fully coordinated Co(H2O)6 can coordinate with hydroxyls without the consideration of electrostatic interaction which I think is more important in terms of stabilizing the precursors. In other words, the discussion is not fully convincing at this moment, the calculation is also not that detailed to understand.

Reply:
We feel great thanks for your professional review work on our article. As you are concerned, there are several problems that need to be addressed. According to your nice suggestions, we have made extensive corrections to our previous draft, the detailed corrections are listed below.
1. First of all, we agree with the reviewer's opinion that the fully coordinated [Co(H2O)6] 2+ was not directly coordinated with the hydroxyl group, instead, it was supposed to be connected by the hydrogen bond (electrostatic interaction) between the oxygen in the silanol groups and the hydrogen in the water of the [Co(H2O)6] 2+ , as shown in Supplementary Fig. 18. Supplementary Fig. 18. The optimized structure of [Co(H2O)6] 2+ , [Co(H2O)6] 2+ -SiO2 and SiO2.
In order to confirm that hydrogen bond stabilized the existence of [Co(H2O)6] 2+ .
Following, the reactions between [Co(H2O)6] 2+ and SiO2 were studied via DFT calculations (the structures were presented in Supplementary Fig. 18). The binding energy (BE) was used to elucidate the interaction between [Co(H2O)6] 2+ and SiO2, which was defined as: The Subsequently, free energy for dehydroxylation of SiO2 and Co/SiO2 were calculated, and the dehydroxylation process of SiO2 was exhibited in Fig. 3h and Supplementary -0.796 eV, respectively. By contrast, the dehydroxylation process of Co/SiO2 was compared to that of SiO2 ( Fig. 3h and Supplementary Fig. 19). Two processes were simulated over Co/SiO2. The first process was [Co(H2O)6] 2+ binding to SiO2 through hydrogen bond, and the second process was dehydroxylation. It is seen that  3h). It is obviously that the rate-determining step in the dehydroxylation over Co/SiO2 was from IM1 to TS, with the energy barrier of 0.634 eV (Fig. 3h), which was lower than that of SiO2. After that, the free energy of IM2 and FS was 0.176 eV and -1.129 eV (Fig. 3h), respectively, suggesting that the Td-Co(II) was acquired spontaneously due to the decrease of free energy. As a result, DFT calculations from Fig. 3h suggested that the energy barrier for dehydroxylation over Co/SiO2 was lower than that over SiO2, confirming that the existence of [Co(H2O)6] 2+ promoted the dehydroxylation on the silica support.

We have added a detailed discussion for the DFT calculation in the revised
Manuscript (line 410-495, page 18-22), by considering that electrostatic interaction between fully coordinated [Co(H2O)6] 2+ and the hydroxyls on the support was important in terms of stabilizing the precursors.
Supplementary Fig. 19. The flow diagram for the dehydroxylation of hydroxyl groups on Dir-reduction catalyst.
Supplementary Fig. 20. The flow diagram for the dehydroxylation of hydroxyl groups on the pure SBA-15 catalyst (To make it easier to show the change of silanol groups, the oxygen was replaced with green atom).
2. Next, we have revised the whole discussions to improve the readability and understandability of the manuscript. In brief, it is evidenced that the cobalt was in the form of [Co(H2O)6] 2+ over Dir-reduction and Dir-reduction(PM) catalysts, which contained abundant absorbed water. In contrast, cobalt nitrate was presented over these Dir-reduction(PM200) and Dir-reduction(PM900) samples, which were lacking in absorbed water.  Reply: Thank you for pointing this out. According to your suggestions, the word of "Co" was replaced by the word of "Co species" in the revised Manuscript (line 193, page 9). (Fig. 1c), the authors said "… all of the Co oxides on the surface were reduced by H2." Therefore, the authors may need to explain that the high content of Co 2+ of the H2-reduction catalyst in Fig.   1b.

Comment(3): According to the result of H2-TPR
Reply: Thanks for your remarks. This phenomenon was mainly caused by the fact that Co oxides in the bulk phase cannot be totally reduced to Co 0 and the different depths of the samples detected by XPS and TPR techniques.
In general, metal oxides loaded on support were difficult to be completely reduced to From the point of view of measuring depth, XPS technique was more likely to detect oxides that were not reduced in the subsurface. Hence, the above difference can be used to explain why there was a visible peak of Co 2+ in both XPS spectra (Fig. 1b) and quasi in-situ X-ray photoelectron spectra ( Supplementary Fig. 5), while no reduction peak was observed in the TPR profiles (Fig. 1c).

Comment(5): In the line 231, the authors said "then mixed physically with
Co(NO3)2·6H2O in glove box before conducting H2-direct reduction  Reply: We really appreciate the careful check on the manuscript. According to the results of H2-TPR and UV-vis in Fig. 2b, d, it is shown that the Co 3+ was present in c PM200, PM900 and IMP900 catalysts, so the XPS spectra of these catalysts should display the peak of Co 3+ . After re-analyze the XPS data, the results were shown in the above figure (Fig. 2c). The peaks at 779.5 eV and 794.6 eV were attributed to the surface Co 3+ species. The peak at 777.5 eV was attributed to the metallic Co. However, in view of the reviewer's comment 7, we have re-characterized the experiments of Fig.   2b and Fig. 2c (the previous experiments of H2-TPR and Co 2p XPS were not carried out under in situ reaction conditions, and the reduced samples were subjected to reoxidation). Notably, the revised Fig. 2b and Fig. 2c are recorded in Comment (7). Reply: Thanks for your remarks. Indeed, the four contrast samples of PM, PM200, PM900 and IMP900 were all conducted by H2-direct reduction treatment (catalyst precursor was reduced directly by H2), and the reduction temperature was 600 o C.
However, the previous experiments of H2-TPR and Co 2p XPS were not carried out under in situ reaction conditions, and the reduced samples were easily re-oxidized in the air. As a result, we speculated that the reduction peak below 600 o C in Fig. 2b from Comment(6) was most likely formed by the surface Co oxides on the reduced samples. In order to eliminate the re-oxidation in the ex situ characterization, quasi in-situ XPS and in-situ TPR were employed, and the revised Fig. 2b and Fig. 2c are indicated as below.
In the revised Fig. 2b (in-situ TPR), Dir-reduction (PM) catalyst exhibited one significant peak higher than 800 o C, representing the reduction of Co(II) species that were strongly interacted with silica support. In contrast, almost no remarkable Revised Fig. 2. (b) in-situ H2-TPR profiles, (c) quasi in-situ Co 2p XPS of the four designed catalysts.
reduction peak was observed in the TPR curves of the Dir-reduction (PM200) and the Dir-reduction (PM900) catalysts, suggesting the absence of Co(II) species that were intensely interacted with SBA-15 support. Significantly, in the Dir-reduction (IMP900) catalyst, the characteristic reduction peak of embedded Co(II) sites on the support was observed, but the temperature and the intensity of the reduction peak at around 750 o C was downshift when compared to the Dir-reduction (PM) sample, revealing that Co species did not interact strongly with the support when the surface OH was insufficient. Furthermore, quasi in-situ XPS of the Dir-reduction (PM) catalyst from the revised Fig. 2c showed the BE of typical high-spin Co(II) species. On the contrary, the rest three catalysts indicated the peaks of metallic Co. In addition, samples were analyzed by the ex-situ UV-vis and recorded in the Fig. 2d (comment (6)). The absorbance at 543 nm, 578 nm, and 642 nm, as the characteristic band for Td-Co(II), were observed in the Dir-reduction (PM) and Dir-reduction (IMP900) catalysts. But the intensity of Td-Co(II) over the Dir-reduction (IMP900) was lower than that on the Dir-reduction (PM). Over the Dir-reduction (PM200) and Dir-reduction (PM900) catalysts, the characteristic peak representing for cobalt oxide at approximately 410 nm and 720 nm were detected. It is worth noting that in the revised Fig. 2b, there was no reduction peak below 600 o C can be found, because these four catalysts were subjected to H2-direct reduction treatment at 600 o C. However, since metallic cobalt was easily re-oxidized in air, ex-situ UV-vis spectra captured the diffraction peak of cobalt oxide (Fig. 2d in comment (6)).
We have revised our manuscript accordingly, and the reduction temperature of the contrast samples (PM200, PM900 and IMP900) has been presented in the revised manuscript (line 232-233, page 10; line 247-248, page 11). Moreover, other revisions can be found from the revised Fig. 2b, Fig. 2c  catalysts. This is attributed to high ratio of Td-cobalt(II) were stabilized on Al2O3.
Therefore, we choose the direct H2-reduction method to treat the catalysts. As a matter of fact, ligand-protected direct hydrogen reduction method has been widely developed in the synthesis of high performance catalyst (Angew. Chem. 2020, 132, 2-11), but the special role of this direct hydrogen reduction method was not clarified in detail.
In our work, the section of "Verifying the effect of absorbed water on Td-Co(II) formation" illustrated the role of adsorbed water for inducing highly dispersed tetrahedral cobalt(II) sites in detail. The direct H2-reduction method was applied to the four contrast samples of Dir-reduction (PM), Dir-reduction (PM200), Dir-reduction (PM900) and Dir-reduction (IMP900). It is noted that when adsorbed water was absent, there was no way to obtain highly dispersed Td-Co(II). Hence, the necessary condition for the availability of highly dispersed Td-cobalt(II) sites was the presence of adsorbed water, rather than the direct H2-reduction treatment.
We have revised our manuscript accordingly, and details can be found in line 516-521, page 23 of the revised manuscript.
Comment(9): In addition, it is better to enlarge the font size of some icons, like Fig. 1(a, b, c, d, e, f).
Reply: Thank you for pointing this out. As suggested by the reviewer, we have enlarged the font size of icons in all the figures.